Not applicable.
Not applicable.
The present invention is directed to a process for cleaning olefinic polymeric fouling from surfaces of reactors, heat exchangers and other process equipment in which polymerized olefins are produced by solution polymerization processes. More specifically, the inventive process is directed to a method whereby the polymeric fouling is dissolved into a high boiling, low vapor pressure aromatic hydrocarbon solvent and removed from the equipment to restore operating efficiency.
During the manufacture of olefinic polymers by solution polymerization such as, for example, polystyrene and the various copolymers of styrene and other olefinic monomers, finished polymer gradually builds up over a period of time on the surfaces of the process equipment where the material is produced reducing the efficiency of the reactions and ultimately requiring such polymer-fouled equipment to be removed from operation; and ultimately requiring that the equipment, under present practice, be transported to a specialized facility where the polymer is literally burned off of the fouled surfaces in specially-constructed furnaces to remove it. In order to transport such polymer-fouled equipment, environmental and safety requirements are such that, before leaving the plant gates, the polymer and equipment must be steamed to remove liquid solvents, usually the medium in which polymerization took place, which may be adhering to the surfaces of the contamination or the equipment itself. This operation occurs at a considerable expense and after the equipment is put back into service, it usually must be again removed for cleaning after an average of only about four months. Thus, it is an object of this invention to provide a process whereby the surfaces of polymeric reactors and heat exchangers which become fouled and contaminated with olefinic polymer buildup can be cleaned without the necessity of transportation from the plant facility. It is also an object of this invention to provide a polymer removal alternative to pyrolysis. It is a still further objective of this invention to lengthen the operation cycles of the equipment between cleaning.
It is an additional objective of this invention to provide a process whereby such reactor surface cleaning may be accomplished by dissolving the polymer in a solvent and removing it as a liquid from the reactor without dismantling equipment or transporting the equipment to off-site facilities.
A still further object of this invention is to provide a flexible process which allows an operator to remove polymer deposits from equipment surfaces at either an on-site or off-site location.
The above objects and other advantages of this invention are accomplished by contacting the fouled surfaces at elevated temperatures with a high boiling, aromatic hydrocarbon solvent having a boiling point above about 400xc2x0 F. and having a low vapor pressure at the solvating temperatures for a particular olefinic hydrocarbon. The process is particularly applicable to the use of some fluids commonly known as thermal fluids which exhibit solvent capacity for the olefinic polymer involved. These thermal fluids are generally aromatic hydrocarbons, normally characterized as being phenylalkyls, biphenyls, or diphenyl ethers, and mixtures thereof. The olefin polymer fouling particularly suited for this application occur from the solution polymerization of monomers having double bonds, which are susceptible to polymerization such as, for example, ethylene, propylene, butylene, butadiene, pentene, pentadiene and styrene, and other vinylidene polymers to cite a few examples.
These olefins are polymerized under solution polymerization conditions in the presence of catalysts which are well known in the art. The hydrocarbon solvents in which the catalytic polymerization reaction occurs are also well known to those in the art to include; for example, cyclohexane, hexane, octane, cyclo-octane and generally oxygen free liquid alkanes or mixtures thereof sold under various trademarks (ExxonMobil""s ISOPAR E is an example). These polymerization reaction solvents have higher vapor pressure solvents which are normally easy to separate from the reaction product polymers. The solution polymerization reaction conditions, solvents, and catalysts are those generally recognized as Ziegler-Natta catalysts and variations thereof, and improvements thereon, continue up through the present.
When polymerized, either as homopolymers, dimer polymers, block co- and terpolymers or heteropolymers that are useful in commerce, a polymeric mass collects which can be dissolved from equipment surfaces through the use of the process of this invention. The contacting of the polymer occurs at elevated temperatures above about 200xc2x0 F., preferably at a temperature of from about 250xc2x0 F. to about 600xc2x0 F. Those versed in the art will recognize that at higher temperatures the driving forces dissolving the polymer will be higher and thus require less contact time. Higher temperature may also cause higher pressure to be required to prevent the residual polymerization solvent or the aromatic high-boiling solvent from boiling where the solvent selection makes this a factor.
One particularly preferred aromatic hydrocarbon solvent is available as a thermal fluid as a mixture of from about 50 wt % to about 66 wt % 1,1-diphenyl-ethane and from about 34 wt % to about 50 wt % of an ethylated benzene sold by Dow Chemical Company under its trademark DOWTHERM(trademark) Q and available as HP Solve(trademark) 515 from Apogee Engineering. The preferred DOWTHERM(trademark) Q solvent mixture has a vapor pressure of about 0.002 mmHg at 25xc2x0 C., making it very desirable for use in the practice of this invention. By xe2x80x9clow vapor pressurexe2x80x9d is meant a vapor pressure below about 0.01 mmHg at 25xc2x0 C. Generally acceptable for the practice of this invention are those aromatic hydrocarbon solvents having a vapor pressure of from about 0.001 to about 0.01 mmHg at 25xc2x0 C.
The process of this invention is practiced by enclosing the fouled equipment surface in either a container designed to fit the equipment or by using the process equipment on the plant facility such as cleaning heat exchanger tubes without removing them from the shell. The equipment may be taken out of service isolated from the rest of the process and connected to a processing system, which heats the treatment fluid to its useful temperature and circulates it through the fouled equipment as mentioned before, preferably a circulation loop. The equipment may also remain in place and existing processing equipment used to circulate and heat the treatment fluid. The heated fluid is pumped into the equipment, such as a reactor or heat exchanger, and is allowed to dissolve at least some of the polymer from the surfaces of the equipment. The solubility of the polymer fouling within the equipment will determine the amount of solvent which must be committed to complete a particular cleaning job and the amount of time required to complete the job. It is contemplated that several cycles of contacting the fouled equipment will be required to completely clean the equipment. It is preferred that the heated solvent be circulated through the equipment to collection tanks where, once it nears saturation with the polymer, it is held for separation, disposal or reuse.
Of course, another aspect of this invention involves the separation of the polymer dissolved in the solvent such that the solvent can be recovered and reused, either in subsequent batch operations or during the circulation and defouling steps. This is accomplished by boiling, or flashing, the solvent in a container under vacuum, usually in the presence of an antifoam agent, which allows the solvent to be separated from the polymer and collected in an additional vessel or by flashing the heated and pressurized mixture into an appropriate vessel. Lower boiling point liquid solvents from the polymerization reaction should be removed from the process equipment before removal of the polymeric olefin fouling deposits. Usually there is some residual reaction solvent present with the polymer. This solvent can also be recovered from the high boiling solvent of this invention and reused. This recovered low boiling point solvent can then be recycled back into the original polymerization process for cost reduction and waste minimization.